Elephant shark takes record for slowest evolution

It's a living fossil to beat all others. The elephant shark, Callorhinchus milii (pictured), has the slowest-evolving genome of any vertebrate.

C. milii is not actually a true shark but belongs to a group known as ratfish, which diverged from sharks about 400 million years ago. When a team led by Byrappa Venkatesh of the Institute of Molecular and Cell Biology in Singapore compared its genome with those of other vertebrates, they found it had changed less from its presumed ancestral form than any other.

C. milii outstrips the coelacanth, the fish that previously held the slow-evolution record.

Such limited change means the elephant shark's genome is the closest yet to that of the first jawed vertebrate, which lived more than 450 million years ago and gave rise to many modern animals including humans. It makes the elephant shark an important reference point for unlocking how this long-lost ancestor evolved. As well as jaws, the earliest fish pioneered bony skeletons and a sophisticated immune system, but it is not known when or how these features appeared.

On the other hand, critics say this view of ratfish as evolving slowly fits poorly with the fossil record, which shows a massive evolutionary radiation in the group between 340 and 300 million years ago. "We're getting this divorce between the apparent conservatism of their genome and the astonishing singing and dancing that was going on anatomically," says Michael Coates, a palaeontologist at the University of Chicago. To resolve the contradiction, he says, biologists need to do similar genomic analyses of other relatively primitive fishes.

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For the elephant shark, evolution is so 400 million years ago (Image: Kelvin Aitken/V and W/Image Quest Marine)